Reservoir Forming Condition Of Lithologic Traps In Mid-upper Section Of Changling Depression, Southern Songliao Basin

The Changling depression of southern Songliao basin is favorable for lithologic reservoir development, and hundred million tons of reserve have discovered recently in the areas of Daqingzijing, Qianan and northwestern Qianan. Further study in geologic background of lithologic trap and the reservoir formation conditions in the Changling depression , especially analysis of reservoir forming mechanism for the Daqingzijing field, are important to guide lithologie reservoir exploration in southern Songliao basin, as well as to expand the theory of reservoir formation of constinental deposit and lithologic reservoir in China.Based on the study of regional petroleum geologic condition, the structural setting and sedimentary facies belts which are favorable for lithologic reservoir formation are recognized. It is considered that the Changling depression is a negative structural element with the characteristics of superimposed basin in which fault depression in the low part and sag in the middle and upper. Later structural reform is weak, and the structural background is favorable for source rock deposition, thermo-evolution and reservoir preservation. Set of petroleum bearing assemblage can be developed in this better sedimentary environment, and various petroleum bearing assemblages were connected each other in vertical and plane view. Southwestern Baokang system consists of fluvial-delta facies deposition, and excellent reservoir rock, developed extensively in delta front belt provide the enough space for petroleum accumulation; the low productive reservoirs are characterized with low porosity, low permeability and inter-bedded with thin shale, and the space of reservoir is mainly inter-granular solution pore-space of the sandstone. In petroleum rich depression delta front sands entered into active source rock were inter-fingering with multiple source rock layers, and the connected area between the sands and coeval source rock is larger than 50 % total area of the source rock. Thus, a group of super-imposed lithologic reservoirs which charactericed by multiple thin inter-bedded petroleum bearing assemblages and larger distribution area was formed, and it made the depression be of the appearance of being full up petroleum. Oscillation of lastrine level is favorable to the formation of various lithologic traps.The reservoir forming model of Gaotaizi syncline structural-lithologic reservoir is summed up through analysis of the case history and reservoir formation. The model can be divided into two types, one is the type for matching relationship of source rock/faults/sands factors and the other is for the matching relationship among the factors of source rock/sands factors developed in confined or semi-confined environment.The prediction of distribution of micro-facies and reservoir rock is the key issue of lithologic reservoir studying in the Changling depression, and combination of multiple information and use of multiple methods is one of the important ways to successful of exploration lithologic reservoir in the Changling depression.The Daqingzijing field is a large composite petroleum bearing area controlled by both structure and lithology. A lot of structural and lithologic reservoirs were developed in it, and the main type is structural-lithologic combined trap; the reservoir formation is controlled by various factors. Depositional debris of the Daqingzijing area came from multiple source areas. The shifting of the channels and the change of lake water body made the sandstone with better continuity in the direction of channel extension, poor continuity in the direction right to the channel extension, rapidly lateral facies variation and poor connection. Thus, it is one of the major factors to make the zone with complicated OWC.